Compacted graphite (CG) irons exhibit a graphite shape intermediate between that of stringy, interconnected flakes in gray iron and the dispersed, disconnected spheroids in ductile iron. In many ways, CG irons combine the better properties of both gray and nodular iron into one material. The yield strength approaches that of ductile iron while the material retains the machining properties and castability of gray iron. CG irons have been recognized as early as 1966 (see U.S. Pat. No. 3,421,886). However, the introduction of commercial CG iron has been inordinately slow.
The chemistry of CG iron is essentially that of nodular iron except that, in processing, the nodularizing agent, such as magnesium, is either added in smaller proportions or is allowed to fade prior to casting, or Ti is added, so that the graphite formation is changed to that of a compacted configuration as opposed to a spheroid. As used herein, "fade" means a diminution in the effectiveness of the nodularizing agent in accordance with the progression of time. The chemistry of a typical nodular iron is 3.2-4.1% carbon, 1.7-2.8% silicon, 0.45-0.8% manganese, 0.1-0.14% phosphorus, 0.05-0.13% sulfur. In a commercial nodular iron, magnesium is used as a treatment element and is retained in the final casting in an amount of about 0.04% and sulfur is reduced to about 0.002%; in a CG iron, the magnesium may be retained in amount of about 0.01-0.03%.
Gray cast iron is the least expensive of all the cast metals. This is due to the type of raw materials used: pig iron, cast iron scrap, steel scrap, limestone, coke and air, all of which are relatively inexpensive. Gray cast iron is commercially used primarily in the as-cast condition, whereas nodular iron (which requires specialized nodularizing treatment) is used in an as-cast, annealed, or normalized condition and, in some cases, it is quenched and tempered.
It is helpful to compare some of the existing or known physical properties of commercial gray iron and commercial nodular iron with known CG irons which have not been significantly commercialized (see Table 1 below).
TABLE 1 ______________________________________ Nodular Gray Iron CG Iron Iron ______________________________________ Tensile Strength (ksi) 22-60 40-70 58-116 Yield Strength (ksi) -- 33-50 36-73 Fracture Elongation (%) 0-.5 2-3 2-15 (at 2% strain) Elastic Modulus 11-17 20-23 23-27 (million psi in tension) Hardness (BHN) 140-270 140-270 140-270 Thermal Conductivity .12-.16 .10-.12 .06-.10 (Cal/cm S .degree.C.) Thermal Expansion 11-12 12-13 11-13 (in/in .degree.C. .times. 10.sup.-6) Shrinkage (relative 1 .9-1.0 .8-1.0 dimensionless unit) Damping (relative 1 .6 .34 dimensionless unit) Casting Yield 60-65% 55-60% 50-55% ______________________________________
It would be extremely desirable if a compacted CG iron could be formulated which continued to exhibit the good physical characteristics of thermal conductivity, shrinkage, and damping similar to that of known CG irons while at the same time have highly improved strength and hardness characteristics approaching that of nodular cast iron. In other words, to approach the combination of characteristics as boxed in Table 1 would be desirable.
The prior art has attempted to increase or optimize certain of the physical characteristics of such iron. In an effort to provide a bainitic/austenitic iron, the prior art has employed the use of certain alloying ingredients, in one case (U.S. Pat. No. 3,860,457) to promote strength characteristic of a bainitic microstructure in nodular iron, and in a second case (U.S. Pat. No. 3,549,431) to promote an increase in thermal expansion in gray iron, also characteristic of a bainitic structure.
In U.S. Pat. No. 3,860,457 a nodular iron was produced (magnesium is 0.03 or greater); the addition of molybdenum and nickel was made to promote pearlite and thereby, in conjunction with the bainite, produce a highly increased strength level. Unfortunately, the use of molybdenum and nickel as pearlite promoters in a nodular iron tends to sacrifice and decrease thermal conductivity, shrinkage and damping, physical characteristics which are of keen interest to this invention. These characteristics are detrimentally injured substantially as a result of the addition of molybdenum and nickel in the amounts recited. It should also be mentioned that molybdenum is generally accepted in the art as a pearlite destroyer during heat treatment, contrary to the teaching of U.S. Pat. No. 3,860,457, and thus the teaching of this patent is suspect.
In U.S. Pat. No. 3,549,431, a gray cast iron was produced which had increased thermal expansion as a result of the addition of elements which included nickel and molybdenum. However, since the thermal expansion proved to be relatively low compared to that of CG irons and, therefore, one cannot deduce that the use of nickel and molybdenum would have any favorable effect upon thermal conductivity, shrinkage or damping now sought to be maintained along with an increase in strength and hardness. In fact, the addition of nickel and molybdenum to a gray cast iron tends to reduce the thermal conductivity, shrinkage and damping characteristics from the levels normally enjoyed with a conventional gray cast iron.